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Re: [Xen-users] SW RAID powered Xen node - knowledgeable feedback much n

To: <office@xxxxxxxxxxx>
Subject: Re: [Xen-users] SW RAID powered Xen node - knowledgeable feedback much needed
From: Luke S Crawford <lsc@xxxxxxxxx>
Date: 19 Dec 2010 20:05:59 -0500
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<office@xxxxxxxxxxx> writes:

> I would like to know your opinion on the viability of a SW RAID 1 powered
> Xen node. The CPU is a i7-920 Quad core, 2.66Ghz, 8MB cache and 8GB of RAM.
> The 2 HDDs are absolutely normal (no raptor, 10k-15k etc - just normal
> SATA) and there's no HW RAID. I was thinking to allocate some 70% of the
> resources to VEs and the rest to the node itself.

Yes. I did fairly extensive tests on hw vs software raid 1 and 1+0
and even with a battery backed cache (only 64MiB at the time, this
was a few years back)  the hw raid was slightly slower than the 
linux MD raid.  It was about the same.   

Now, with a larger (and battery-backed) cache and raid5 or 6 rather
than raid 1 or 1+0, the story may be different, but if you only
have two spindles, in my opinion, it's better to spend the money
on more spindles.

Me, all my servers have 8 cores (a single 2.0ghz G34 opteron) 
32GiB of registered ecc ram and 4 "enterprise" sata disks in a 1+0 
linux MD raid.

The big thing you've gotta watch for are people who use a bunch 
of disk IO.  even one guest can make I/O suck for a lot of people
(and when I have better tools for doing that, I'll post them here. 
Ideas are appreciated.)  

but overall, my system seems to work fairly well, even when I put 31GiB
worth of 512MiB, 256MiB, 128MiB and 64MiB guests on the same host.  
I/O does suck, but I don't think we're going to get around that
without spending a whole lot on a raid card.

I have been looking at bcache[1]... when it's done, it will be 
pretty awesome, but I don't know how "done" it is just yet. 


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